Optical harmonic generation using a non-linear medium such as a crystal, provides a method for doubling, tripling or quadrupling of the frequency of electromagnetic radiation emitted by a laser or other high intensity source. Harmonic generation and optical parametric oscillation have been demonstrated in crystals such as LiNbO, KDT, KTP, BBO, LBO and others. Blue, green and UV lasers have found industrial, medical and scientific applications.
UV lasers are used in drilling, microwelding and the like. Intracavity fourth harmonic generation has been suggested. See Tso Yee Fan and Bruce H. T. Chai, "Intracavity Fourth Harmonic Generation Using Three Pieces of LBO in a Nd:YAG Laser", OSA Proceeding on Advanced Solid State Lasers 1994 Vol. 20, pps. 377-380. However, such prior art fourth harmonic lasers are lower powered in the order of about 270 mw of fourth harmonic power. Other fourth harmonic lasers have been proposed see U.S. Pat. No. 5,206,868. There still exists a need for improved high powered fourth harmonic lasers.